The treating of carbon black with polymeric binders is disclosed in the art. Previous patents disclose the art of incorporating additives to produce pelletized treated filler materials, in particular pelletized treated carbon blacks, with improved handling characteristics, for example, low dust.
The construction of insulated electrical conductors, i.e., wire and cables designed for medium and high voltage applications, is known in the art. Typical constructions include a core conductor which comprises one or more strands of a conducting metal or alloy such as copper or aluminum; a layer of a semiconductive shielding compound; a layer of insulation such as crosslinked polyethylene or ethylene-propylene rubber and a layer of a semiconductive insulation shield compound overlaying the insulation.
The conductor shield, the insulation shield and the overlaying semiconductive shield layer may be formed by either a two pass or by a single pass triple extrusion process. A two pass operation refers to a process whereby the conductor shield and the insulation layer are extruded in tandem and then crosslinked prior to extrusion of the semiconductor insulation layer. A single pass triple extrusion process refers to a process in which the conductor shield, the insulation layer and the semiconductive shield are all extruded in a common extrusion head and crosslinked simultaneously. The single pass triple extrusion process minimizes production steps and hence is a preferred method of manufacture. However, the single pass triple extrusion process generally makes the semiconductive shield layer more fully bonded to the insulation layer, than in a two pass operation.
Generally, in order to splice insulated electrical wires, or make terminal connections, the semiconductive shield layer should be stripped from the insulation layer. Stripping the semiconductive shield layer from the insulation shield layer is often very difficult. In a situation where the semiconductive shield layer contains carbon black, a carbon containing residue on the surface of the insulation shield often results. The carbon residue may disadvantageously promote treeing in the insulation layer which will ultimately lead to electrical breakdown of the cable. It is therefore advantageous and desirable for a semiconductive shield layer to have a low strip force (be easily separable) when being removed from the insulation layer and for the semiconductive shield layer to leave minimal amounts of carbon residue on the surface of the insulation shield.
Strippable conductive shield compositions are those which can be separated from a crosslinked insulation layer without leaving appreciable amounts of residue on the insulation layer. Usually, the force required to separate a strippable conductive shield composition is significantly lower than the separation force required for bonded shield compositions.
There is a significant cost difference between strippable and bonded semiconductive shield compositions based on existing technological approaches. It would be advantageous, to produce more cost effective strippable formulations than those developed from the technical approaches utilized to date.